The basic problem of stopping a rotating shaft or connecting two shafts having an initial differential speed, involves the dissipation of the friction heat generated in the stopping or connecting process. Conventional clutches and brakes employ multiple disc devices for this purpose, half of the discs being attached to the input shaft and half to the output. Pairs of discs are assembled alternately, one being a plain steel disc having smooth surfaces, and the other having a friction material (such as sintered bronze or paper) bonded to each side. These friction surfaces are grooved in varous ways to facilitate oil circulation, the breaking up of the contact surface into small areas incapable of sustaining thrust load, and providing cutting edges to penetrate the oil film quickly.
There are two basic problems with cooling clutches and brakes. Because oil is normally supplied to the inside radius of the discs to then flow outward, it is impossible to cool the contacting surfaces uniformly, because the amount of oil passing through the disc is fixed by the area of the oil passages at the inner radius, while the contact area itself is increasing by the square of the radius as one proceeds outward.
When normal failure occurs it is usually the steel disc that succumbs first, because intense heat is being applied to both sides, whereas the friction disc has the benefit of more intimate oil contact on both sides (groove wall area) with either a heat insulator (paper) or a better heat conductor (sintered bronze) protecting the steel core.
The patents referred to above discuss the problems of disc failure and suggest the use of perforated discs with oil circulating axially, from end to end.
Whatever design technique is used, it is desirable that: (1) the individual contact areas between the discs should be small (about 0.003-0.005 in.sup.2), uniform and numerous; (2) the oil-wetter perimeters of the individual contact surface areas should be maximized (for example, about 60-70 in/in.sup.2); (3) the ratio of oil supply to contact area should be uniform over the entire disc surface; (4) the cooling capacity should be essentially the same for both friction and steel discs.